Polyethylene resins include various kinds of resins, such as high-pressure low-density polyethylene resins, high-density polyethylene resins and linear low-density polyethylene resins including ethylene/α-olefin copolymers.
Of these, the low-density polyethylene resins are excellent in the balance between pliability and heat resistance and, therefore, are widely used for gaskets of injection molding machines, and for various packing, tube and sheet materials.
The conventional low-density polyethylene resins are excellent in heat resistance but do not have sufficient pliability. Therefore, improvement in the properties has been desired.
For example, a method of blending various elastomers (e.g., ethylene/propylene copolymer rubber and ethylene/1-butene copolymer rubber) with the low-density polyethylene resin to give pliability to the resins has been proposed.
However, such a method has a drawback in that, when the conventional ethylene elastomer is blended with the low-density polyethylene resins, the pliability of the resins is improved but the heat resistance thereof becomes remarkably worse.
Accordingly, now desired is development of the polyethylene resin composition capable of providing moldings being well-balanced in pliability and heat resistance, and exhibiting an improved flowability in various molding processes.
Among the polyethylene resins as mentioned above, for example, the high-pressure low-density polyethylene resins, the high-density polyethylene resins and the linear low-density polyethylene resins including ethylene/α-olefin copolymers have been molded into films and widely used in various fields, such as in packaging of products from the past.
Specifically, the films of the linear low-density polyethylene resins including ethylene/α-olefin copolymers are used for sealants of various packaging materials, because the linear low-density polyethylene resins as film materials can be made at low energy consumption (i.e., small production cost) of production, as compared with the conventional high-pressure low-density polyethylenes, and are excellent in heat-sealability through contaminants, hot tack and mechanical properties, such as tear strength and impact strength.
Though the suitability for high-speed filling upon packaging by automatic filling machines is required for the purpose of packaging, the film made singly from the linear low-density ethylene/α-olefin copolymer does not always sufficiently satisfy the required handling characteristics and suitability for high-speed filling.
Accordingly, now desired is development of the resins capable of providing films having more excellent low temperature heat-sealing properties, sealing stability, slip characteristics and antiblocking properties.
The linear low-density ethylene/α-olefin copolymer is low in melt tension for its molecular weight, as compared with the high-pressure polyethylene. Therefore, it has a drawback in that, when they are molded into films at high speed in inflation molding, rocking or breakage of bubbles is liable to occur. The linear low-density ethylene/α-olefin copolymer has a further drawback in that, since the number of branchings in its molecular chain is small, the flowability thereof in high-shear region becomes worse.
Accordingly, in order to solve the above problems, many kinds of compositions or films have been proposed. For example, there have been proposed a composition made by blending a low-crystalline ethylene copolymer having a density of not more than 0.905 g/cm3 with the ethylene/α-olefin copolymer (Japanese Patent L-O-P No. 34145/1982) and a composition made by blending an ethylene/vinyl acetate copolymer with the ethylene/α-olefin copolymer (Japanese Patent L-O-P No. 109543/1984).
However, films of these compositions could not solve the above problems and, therefore, there is room for improvement in melt tension, flowability in high shearing region, low temperature heat-sealing properties, mechanical properties including tear strength, transparency, and antiblocking properties.